Treating laser radiation is used in laser surgical methods, in particular, for operations in optical surgery. In this connection, the treating laser radiation is focused within the tissue, i.e. beneath the tissue surface, such that an optical break-through is created in the tissue. The treating laser radiation acts through photo disruption or photo ablation.
In the tissue, several processes initiated by the treating laser radiation occur one after the other. If the power density of the radiation exceeds a threshold value, an optical break-through will result, which generates a plasma bubble in the tissue. Said plasma bubble grows after creation of the optical break-through due to expanding gases. If the optical break-through is not maintained, the gas generated in the plasma bubble is absorbed by the surrounding tissue, and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a tissue boundary, which boundary may quite well be located within a tissue structure as well, tissue will be removed from said boundary. Therefore, this is then referred to as photo ablation. In connection with a plasma bubble which separates tissue layers that were previously connected, one usually speaks of photo disruption. For the sake of simplicity, all such processes are summarized here by the term optical break-through, i.e. said term includes not only the actual optical break-through, but also the effects resulting therefrom in the tissue.
For a high accuracy of a laser surgical method, it is indispensable to guarantee high localization of the effect of the treating laser beams and to avoid, if possible, collateral damage to adjacent tissue. It is, therefore, common in the prior art to apply the treating laser radiation in a pulsed manner, so that the threshold value for the power density of the treating laser radiation required to cause an optical break-through is exceeded only during the individual pulses. In this regard, U.S. Pat. No. 5,984,916 clearly shows that the spatial area of the optical break-through (in this case, of the generated interaction) strongly depends on the pulse width. Therefore, high focusing of the laser beam in combination with very short pulses allows placing of the optical break-through in a tissue with in a very punctiform manner.
The use of pulsed treating laser radiation was recently established in ophthalmology, particularly for correction of visual deficiencies. Visual deficiencies of the eye often result from the fact that the refractive properties of the cornea and of the lens do not cause proper focusing on the retina. In the case of nearsightedness (also referred to as myopia), the focus is located in front of the retina when the eye is relaxed, whereas in the case of farsightedness (also referred to as hyperopia), the focus is located behind the retina.
U.S. Pat. No. 5,984,916 mentioned above, as well as U.S. Pat. No. 6,110,166, describe methods of correcting visual deficiencies by means of suitably generating optical break-throughs, so that, ultimately, the refractive properties of the cornea are selectively influenced. A multitude of optical break-throughs are joined such that a lens-shaped partial volume is isolated within the cornea of the eye. The lens-shaped partial volume which is separated from the remaining corneal tissue is then removed from the cornea by means of a laterally opening cut. The shape of the partial volume is selected such that, with the volume removed, the refractive properties of the cornea are changed so as to generate the desired correction of visual deficiency.
In order to isolate the partial volume, it is indispensable, of course, to generate the optical break-throughs at predetermined sites. U.S. Pat. No. 5,984,916 describes corresponding sensors which sense the cross-section as well as the position and intensity of the treating laser beam and feed a corresponding control unit, which influences the laser treatment beam such that an optical break-through is achieved at the desired target point.
In contrast thereto, EP 1,232,734 A1 suggests to use a wavefront sensor to determine the position of an optical break-through which has been generated in an eye. In this connection, said publication states that the bubble size can be measured by the wavefront sensor using “relatively well-known wavefront techniques”. Unfortunately, this document remains silent as to how such measurement could be effected. It does not convey any technical teaching, but merely states a problem to be solved. However, since a wavefront sensor is known to be capable of determining a distortion of a wavefront, it would be conceivable to achieve the object mentioned in EP 1,232,734 A1 as the problem to be solved by using a wavefront sensor to detect a deformation of the corneal front surface created by a bubble generated inside the cornea. However, in such a conceivable approach it would have to be expected that the precision of measurement strongly decreases as the size of the bubble decreases, because a smaller bubble will certainly also lead to a strongly reduced deformation of the front surface of the cornea. Moreover, it would have to be expected that, for a bubble located at a greater depth, the bubble diameter indicated by this type of measurement would be smaller than for a bubble of the same size located higher up. Thus, in such an approach an error of measurement as a function of the depth position of the bubble would have to be expected.
The precision with which treating laser radiation can be applied to a patient's eye to correct a visual deficiency, for example, naturally has a direct effect on the quality of the result, i.e. in the example mentioned, on the quality of an optical correction. Therefore, it is an object of the invention to provide a device for measuring an optical break-through by which an increased selectivity of the effect of treating laser radiation can be achieved.